He allowed himself a moment of triumph. He was one step closer to his goal of crippling the Resistance in northern France. He had pulled it off despite the stupidity of the Gestapo. And it had been exhilarating.
The challenge now was to make maximum use of Helicopter's trust. The agent must continue to operate, believing himself unsuspected. That way he could lead Dieter to more agents, perhaps dozens more. But it was a subtle trick to pull off.
They arrived at the rue du Bois and Stephanie drove into Mademoiselle Lemas's garage. They entered the house by the back door and sat in the kitchen. Stephanie got a bottle of scotch from the cellar and poured them all a drink.
Dieter was desperately anxious to confirm that Helicopter had a radio. He said, "You'd better send a message to London right away."
"I'm supposed to broadcast at eight p.m. and receive at eleven."
Dieter made a mental note. "But you need to tell them as soon as possible that the cathedral rendezvous is compromised. We don't want them to send any more men there. And there could be someone else on his way tonight."
"Oh, my God, yes," the young man said. "I'll use the emergency frequency."
"You can set up your wireless right here in the kitchen."
Helicopter lifted the heavy case onto the table and opened it.
Dieter hid a sigh of profound satisfaction. There it was.
The interior of the case was divided into four: two side compartments and, in the middle, one front and one back. Dieter could see immediately that the rear middle compartment contained the transmitter, with the Morse key in the lower right-hand corner, and the front middle was the receiver, with a socket for headphone connections. The right-side compartment was the power supply. The function of the left-side compartment became clear when the agent lifted the lid to reveal a selection of accessories and spare parts: a power lead, adaptors, aerial wire, connection cables, a headset, spare tubes, fuses, and a screwdriver.
It was a neat, compact set, Dieter thought admiringly; the kind of thing the Germans would have made, not at all what he would expect from the untidy British.
He already knew Helicopter's times for transmission and reception. Now he had to learn the frequencies used and-most important-the code.
Helicopter plugged a lead into the power socket. Dieter said, "I thought it was battery-operated."
"Battery or mains power. I believe the Gestapo's favorite trick, when they're trying to locate the source of an illicit radio transmission, is to switch off the town's electricity block by block until the broadcast is cut off."
Dieter nodded.
"Well, with this set, if you lose the house current, you just have to reverse this plug, and it switches to battery operation."
"Very good." Dieter would pass that on to the Gestapo, in case they did not already know.
Helicopter plugged the power lead into an electrical outlet, then took the aerial wire and asked Stephanie to drape it over a tall cupboard. Dieter looked in the kitchen drawers and found a pencil and a scratch pad that Mademoiselle Lemas had probably used to make shopping lists. "You can use this to encode your message," he said helpfully.
"First I'd better figure out what to say." Helicopter scratched his head, then began to write in English:
ARRIVED OK STOP CRYPT RENDEZVOUS UNSAFE STOP NABBED BY GESTAPO BUT GOT AWAY OVER
"I suppose that's it for now," he said.
Dieter said, "We should give them a new rendezvous for future incomers. Say the Cafe de La Gare next to the railway station."
Helicopter wrote it down.
He took from the case a silk handkerchief printed with a complex table showing letters in pairs. He also took out a pad of a dozen or so sheets of paper printed with five-letter nonsense words. Dieter recognized the makings of a one-time-pad encryption system. It was unbreakable-unless you had the pad.
Over the words of his message, Helicopter wrote the five-letter groups from the pad; then he used the letters he had written to select transpositions from the silk handkerchief. Over the first five letters of ARRIVED he had written the first group from his one-time pad, which was BGKRU. The first letter, B, told him which column to use from the grid on the silk handkerchief. At the top of column B were the letters Ae. That told him to replace the A of ARRIVED with the letter e.
The code could not be broken in the usual way, because the next A would be represented not by E but by some other letter. In fact, any letter could stand for any other letter, and the only way to decrypt the message was by using the pad with the five-letter groups. Even if the code breakers could get hold of a coded message and its plain-language original, they could not use them to read another message, because the next message would be encoded with a different sheet from the pad-which was why it was called a "one-time" pad. Each sheet was used once, then burned.
When he had encrypted his message, Helicopter flicked the on/off switch and turned a knob marked in English "Crystal Selector." Looking carefully, Dieter saw that the dial bore three faint markings in yellow wax crayon. Helicopter had mistrusted his memory and had marked his broadcast positions. The crystal he was using would be reserved for emergencies. Of the other two, one would be for transmission and the other for reception.
Finally he tuned in, and Dieter saw that the frequency dial was also marked with yellow crayon.
Before sending his message, he checked in with the receiving station by sending: HLCP DXDX QTC1 QRK? K
Dieter frowned, figuring. The first group had to be the call sign "Helicopter." The next one, "DXDX," was a mystery. The number one at the end of "QTC1" suggested that this group meant something like: "I have one message to send you." The question mark at the end of "QRK?" made him think this asked if he was being received loud and clear. "K" meant "Over," he knew. That left the mysterious "DXDX."
He tried a guess. "Don't forget your security tag," he said.
"I haven't," Helicopter said.
That must be "DXDX," Dieter concluded.
Helicopter turned to "receive" and they all heard the Morse reply: HLCP QRK QRV K
Once again, the first group was Helicopter's call sign. The second group, "QRK," had appeared in the original message. Without the question mark, it presumably meant "I am receiving you loud and clear." He was not sure about "QRV," but he guessed it must mean "Go ahead."
As Helicopter tapped out his message in Morse, Dieter watched, feeling elated. This was the spy catcher's dream: he had an agent in his hands and the agent did not know he had been captured.
When the message was sent, Helicopter shut down the radio quickly. Because the Gestapo used radio direction-finding equipment to track down spies, it was dangerous to operate a set for more than a few minutes.
In England, the message had to be transcribed, decoded, and passed to Helicopter's controller, who might have to consult with others before replying; all of which could take several hours, so Helicopter would wait until the appointed hour for a response.
Now Dieter had to separate him from the wireless set and, more importantly, from his coding materials. "I presume you want to contact the Bollinger circuit now," he said.
"Yes. London needs to know how much of it is left."
"We'll put you in touch with Monet, that's the code name of the leader." He looked at his wristwatch and suffered a moment of sheer panic: it was a standard issue German Army officer's watch, and if Helicopter recognized it the game would be up. Trying to keep the tremor out of his voice, Dieter said, "We've got time, I'll drive you to his house."